1. Field of the Invention
The present invention relates to a system and method for adapting spreading codes to the transmission channel medium in communication networks. Specifically, the present invention relates to a system and method for selecting an optimum spreading code for Direct-Sequence Spread-Spectrum (DSSS) node communication, based upon information gathered about the radio-frequency link between nodes, such that interpath interference is minimized.
2. Description of the Related Art
Many communication systems employ the use of spreading codes to transmit signals, such as voice, data or multimedia signals between transceivers, or nodes, of a network. In such applications, the narrow-band data transmission signal within the single frequency band is multiplied by a spreading code having a broader band than the user data signal and the user data signal is “spread” to fill the entire frequency band used. As discussed in U.S. Pat. No. 5,515,396 issued to Michael D. Kotzin, which is incorporated herein by reference, the modulation of a signal to be transmitted often includes taking a baseband signal (e.g., a voice channel) having a bandwidth of only a few kilohertz, and distributing the signal to be transmitted over a frequency band that may be many megahertz wide. Although spreading the user data signal may be accomplished by several methods, the most common is to modulate each bit of information, generally after appropriate error correction coding, with a spreading code sequence of bits. In doing so, many bits are generated for each coded information bit that is desired to be transmitted. However, transmissions between transceivers are subject to interference from a number of sources, therefore corrections and compensations must be considered when implementing Direct-Sequence Spread-Spectrum (DSSS) systems.
One such source of interference is multipath propagation between transceivers. Signals propagated along different paths, arrive at the receiver at different times due to variations in transmission delays. As discussed in U.S. Pat. No. 5,677,934, issued to Kjell Ostman, which is incorporated herein by reference, multipath propagation profiles are highly dependent upon the environment of the communication link. As pointed out in Ostman, when the signaling period is long, and delayed copies of the transmitted signals are received with a delay that is long in comparison to the signaling period, multipath propagation compensation is required. Furthermore, in mobile networks, the communications receiver sees rapid changes in phase and amplitude of a received signal and is required to track such changes. Several methods exist for addressing these effects, such as the use of RAKE receivers to collectively assemble transmitted signals. Through the use of a RAKE-receiver algorithm, a complete transmission may be derived from the multiple propagated signals within the receiver.
Multipath interference can also be cancelled through the use of spreading codes when carefully selected to ensure that secondary multipath rays are concurrent with the lowest sidelobe values of the selected spreading code's periodic autocorrelation functions. Accordingly, a need exists for a system and method for the selection of spreading codes for communication, based on information gathered between two nodes, in order to minimize and preferably eliminate multipath interference.